1. Field of the Invention
This invention relates to a redundant switched full-duplex Ethernet type communication network, particularly in avionics.
2. Description of the Related Art
The Ethernet network, which is the reference in the world of communication networks, can be used to send data in digital form by packets or “frames”, where a frame is defined as being a set of data sent in a single step on the network.
In an Ethernet network, the data in each frame are not interpreted. The network carries the data without understanding their meaning. A frame is composed of two types of data, network data that are used to route the frame to its correct destination, and useful data which comprise the “useful load” in the frame.
An Ethernet network is composed of different equipment that is subscribed to the network, and connected to each other through a communication means formed of active equipment called switches, which perform three functions:                connect network subscribers in point to point mode through physical links, which are physical supports for messages to be exchanged, for example twisted pair cables,        route (switch) frames sent by source equipment to one or more destination equipment,        check the integrity and the format of the Ethernet frame.        
FIG. 1 illustrates an Ethernet network composed of two switches 11 interconnected to each other and each connected to three items of subscriber equipment 12 in point to point mode.
Operation of such a network is simple. Each network subscriber can send frames in digital form at any time towards one or several other subscribers. When a switch receives the frames, the “network information” data are analyzed to determine the destination equipment. The frames are then switched towards this equipment.
In the “switched full-duplex Ethernet type network” expression:                the “full-duplex” term means that the subscriber can send and receive frames at the same time on the same link,        the “switched” term means that the frames are switched in switches on appropriate outputs.        
For example, this network may be a 100 Mbits/s switched full duplex type network on twisted pair; the term “twisted pair” means that connections between the equipment and the switches are composed of two pairs of cables, each pair being twisted; the term 100 Mbits/s simply means the transmission or reception speed of frames on the network.
The Ethernet technology imposes:                a minimum size and a maximum size on the frames,        an identification of the source and/or the destination(s) in each frame,        a CRC (“Cyclic Redundancy Check”) that checks the integrity of the transported data.        
At the present time, in the civil aeronautics field, data exchanges between the various onboard computers are based on the use of the ARINC 429 aeronautical standard.
However, the switched full-duplex Ethernet network is frequently used in industry. The emergence of new communication technologies shows that this type of network is an open and standard solution (IEEE standard 802.3) with a considerable potential for development as a local network. But this type of solution does not provide any means of guaranteeing segregation and transfer performances (in terms of throughput, latency, etc.) necessary for avionics applications.
The purpose of this invention is to improve the availability of such a network, by providing means of protecting against the loss of a link or a switch, to enable its use in avionics.
In general, in a distributed control system, the communication system is made redundant so that each node in a set of nodes (equipment) can be controlled from one of these nodes, and so that these nodes can be connected so as to increase the load factor on the system and therefore its efficiency.
A European patent application EP-0 854 610 describes an Ethernet communication redundancy process between a set of nodes forming such a distributed control system. These nodes are connected to each other in duplex through communication lines in a first and a second Ethernet network, that are independent from each other. At the transmission end, a first node transmits identical data on communication lines in the first and the second networks, a data identifier being added to the transmitted data. A second node at the reception end determines which of the identical data received from the first node through communication lines in the first and the second networks arrived first and uses it as the reception data. The second data are then rejected if they are identical to the first data.
Unlike the process described in this patent application which is applicable to data, the purpose of the invention is to make a process enabling frame by frame redundancy.